<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>后盾人</title>
    <style>
        * {
            padding: 0;
            margin: 0;
        }

        body {
            padding-left: 100px;
            padding-top: 100px;
        }

        main {
            position: relative;
            width: 400px;
            height: 400px;
            border: solid 5px blueviolet;
        }

        main header {
            position: fixed;
            width: 200px;
            height: 200px;
            background: red;
            left: 0px;
            top: 0px;
        }
    </style>
</head>

<body>
    <main>
        <header></header>
    </main>
</body>

</html>